A typical window 100 of the prior art is shown in FIG. 1. The window 100 may include one or more panes of glass 110, which may be embedded in a single sash, or in an upper and lower sash such as in a double-hung window. The sash is secured in a frame 120, which consists of two side jambs 130, a top jamb 140, and a sill 150. The window frame 120 is typically made from wood, vinyl, aluminum, or fiberglass, but may be made from any durable, rigid material.
Typically, a window is installed into a rough opening 200 in a house or building, as shown in FIG. 2. The rough opening 200 forms a window cavity 202 surrounded by a header 210, two sides 220, and a sill 230. The header 210 must be constructed sufficiently sturdy to support the necessary roof loads, since these loads cannot be supported by the window unit 100. This is especially important with large window units 100, or when a “window wall” is created with multiple windows side-by-side. The rough opening 200 has an interior side 240 and an exterior side 250 relative to the building itself. The sill 230 is sloped toward the exterior side 250 to allow water that makes its way to the sill 230 to drain out the exterior of the building. The height and width of the window cavity 202 is constructed larger than the height and width of the window frame 120; typically about three-quarters of an inch (approximately two centimeters) larger in each direction. This leaves an approximately three-eighth inch space (about one centimeter) between the window 100 and the rough opening 200 on each of the four exterior faces 160 (the top 120, sill 150, and both sides 130) of the window 100.
To hold the window unit 100 in place, the unit 100 is generally constructed with a nailing or installation flange 170 near the exterior edge on each of the four faces 160 of the window frame 120. FIG. 3 shows the window 100 of FIG. 1 sectioned along line 3-3, and shows the relationship of the nailing flange 170 versus the rest of the window frame 120 and the glass 110. FIG. 4 shows the same section of window 100, this time with the nailing flange 170 being used to secure the window frame 120 to one of the sides 220 of the rough opening 200. The window 100 is installed so that the nailing flange 170 is on the building exterior 250. Nails 300 are then placed through both the flange 170 and the side 220 of the rough opening 200. These nails 300 are used around the circumference of the window 100, preferably centering the window 100 in the opening 200.
Because the opening 200 is deliberately sized larger than the window 100, a space 310 is created between the opening 200 and the window. Modern construction techniques involve creating a vapor barrier between warm moist air inside a house and the outside, cooler air. To complete the vapor barrier, it is necessary to extend the vapor barrier from the rough opening 200 of the house framing to the window 100 itself. To accomplish this, foam 320 is inserted into space 310 around all four faces 160 of window 100. This foam 320 also serves to insulate this gap 310. Most window manufacturers carefully advise the window installers to take steps to prevent the expanding foam 320 from warping the window frame 120. In most cases, installers are instructed to use low expanding foam 320. In addition, installers are instructed to begin inserting the foam 320 at the nailing flange 170, but to avoid filling the entire space 310 all the way to the interior 240 of the rough opening 200 and window frame 120. This should allow the expansion of the foam 320 within space 310 without warping the window frame 120.
To prevent water leakage under the nailing flange 170, installers will generally place a sealant between the flange 170 and the exterior surface 250 of the rough opening 200. Sill flashing is used on the sill 230 to provide a moisture barrier to prevent water that enters the window cavity 202 after installation of the window 100 from entering the wall under the sill 230. Moisture in the window opening 202 will ideally pool on the sill flashing, where it will generally drain down the non-wood side of the exterior building paper. Any moisture that does not drain off the sill will remain on the sill flashing until it evaporates. Because of this, it is generally encouraged that sealant not be used on the bottom or sill nailing flange 170, in order to allow for drainage and evaporation from outside.
Unfortunately, this prior art technique of window construction and installation has caused various moisture and mold problems in today's buildings. What is needed is an improved construction and installation method for windows the does not cause these problems.